Many types of cellular wireless communication systems are known in the art. When a mobile station, or user equipment (UE), powers up, it is unaware of which cell, or even system, in which it is operating. The UE initiates a search of predetermined frequencies, searching for known synchronization signals. Once the synchronization signals are found and decoded, the UE searches for a control channel and downloads system- and cell-specific information.
The Universal Mobile Telecommunications System (UMTS) is a third-generation (3G) wireless communication technology. The UMTS access network is the UMTS Terrestrial Radio Access Network (UTRAN). Long Term Evolution (LTE) is an evolution of UMTS. The access network in LTE is called Evolved UTRAN (E-UTRAN). E-UTRAN will operate over a very wide span of operating bandwidths and carrier frequencies, in both Frequency Division Duplex (FDD) and Time Division Duplex (TDD) systems. The LTE air interface utilizes Orthogonal Frequency Division Multiple Access (OFDMA) in the downlink and Single-Carrier FDMA (SC-FDMA) in the uplink. LTE systems are anticipated to operate from micro cells up to macro cells with 100 km cell range. The OFDMA radio access technology adapts well to a variety of different propagation conditions, as will be required to handle the different radio conditions that may occur in LTE systems.
To facilitate the cell searching function necessary for mobile UE, two synchronization signals are transmitted periodically every 5 ms in a cell: a primary synchronization signal (P-SCH) and a secondary (S-SCH). The synchronization signals carry information on the physical layer cell identity, a unique (in a wide geographical area) identity of the cell.
Three versions are defined for the P-SCH, one for each of three cell identities within one out of 504 groups of cells. The three versions of P-SCH are common to all cell groups. Since there are only three versions, the straight-forward, prior art approach to P-SCH detection is to conduct matched filtering over at least 5 ms of received samples for each of the P-SCH versions, in order to identify correlation peaks that may reveal synchronization signals from one or more cells. Once a cell candidate has been found, the location of S-SCH can be hypothesized based on the position of P-SCH. Hypothesized, since the position of S-SCH differs depending on duplex mode (FDD/TDD) and cyclic prefix length.
The matched filtering approach is straight-forward to apply in case of FDD, where uplink (UL) and downlink (DL) transmissions occur on different radio channels—that is, UL and DL are separated in frequency. The case of TDD, however, presents a greater challenge, as the same radio channel is used for UL and DL. Whether a subframe is allocated for UL or DL depends on how TDD cells on that particular carrier frequency are configured. FIG. 5 depicts a TDD frame structure (the figure is taken from FIG. 4.2-1 of 3GPP Technical Specification 36.211, “3rd Generation Partnership project; Technical Specification Group Radio Access Network; Evolved Universal Terrestrial Radio Access (E-UTRA); Physical Channels and Modulation (Release 8),” V8.4.0, Section 6.11). A subframe in the TDD frame may be allocated to either UL or DL traffic. P-SCH is transmitted in positions marked DwPTS. Table 1 depicts the UL/DL configuration options (taken from Table 4.2-2 of the same specification), wherein U denotes UL traffic, D denotes DL traffic, and S is switched (both UL and DL).
TABLE 1Uplink-downlink allocation on subframe basis for different configurations in TDDUplink-downlinkDownlink-to-UplinkSubframe numberconfigurationSwitch-point periodicity012345678905 msDSUUUDSUUU15 msDSUUDDSUUD25 msDSUDDDSUDD310 ms DSUUUDDDDD410 ms DSUUDDDDDD510 ms DSUDDDDDDD65 msDSUUUDSUUD
The first time a TDD carrier is visited, during initial cell search or inter-frequency cell search, the synchronization (timing) used on that particular carrier is unknown. When the UE is searching for cells, it must search over the full 5 ms interval. Some of the subframes over which the search is conducted will be allocated for UL, and others for DL. If a nearby UE transmits on its UL, this may cause interference at a receiver that is orders of magnitude larger than the signal transmitted from the network (i.e., a base station, known in E-UTRAN as an evolved Node B (eNB)).
A further complication in initial cell search is that since no signaling is available, the UE is unaware of which duplex mode to expect on a particular carrier frequency. Table 2 depicts the frequency bands that are supported in E-UTRAN, along with duplex mode(s) used in each particular band (the figure is taken from Table 5-2.1 of 3GPP Technical Specification 36.101, “3rd Generation Partnership project; Technical Specification Group Radio Access Network; Evolved Universal Terrestrial Radio Access (E-UTRA); User Equipment (UE) radio transmission and reception (Release 8),” V8.3.0, Section 5.2).
TABLE 2E-UTRAN frequency bands and supported duplex modesUplink (UL)Downlink (DL)eNode B receiveeNode B transmitE-UTRAUE transmitUE receiveDuplexBandFUL—low-FUL—highFDL—low-FDL—highMode11920 MHz-1980 MHz2110 MHz-2170 MHzFDD21850 MHz-1910 MHz1930 MHz-1990 MHzFDD31710 MHz-1785 MHz1805 MHz-1880 MHzFDD41710 MHz-1755 MHz2110 MHz-2155 MHzFDD5824 MHz-849 MHz869 MHz-894 MHzFDD6830 MHz-840 MHz875 MHz-885 MHzFDD72500 MHz-2570 MHz2620 MHz-2690 MHzFDD8880 MHz-915 MHz925 MHz-960 MHzFDD91749.9 MHz-1784.9 MHz1844.9 MHz-1879.9FDDMHz101710 MHz-1770 MHz2110 MHz-2170 MHzFDD111427.9 MHz-1452.9 MHz1475.9 MHz-1500.9FDDMHz12698 MHz-716 MHz728 MHz-746 MHzFDD13777 MHz-787 MHz746 MHz-756 MHzFDD14788 MHz-798 MHz758 MHz-768 MHzFDD. . .17704 MHz-716 MHz734 MHz-746 MHzFDD. . .331900 MHz-1920 MHz1900 MHz-1920 MHzTDD342010 MHz-2025 MHz2010 MHz-2025 MHzTDD351850 MHz-1910 MHz1850 MHz-1910 MHzTDD361930 MHz-1990 MHz1930 MHz-1990 MHzTDD371910 MHz-1930 MHz1910 MHz-1930 MHzTDD382570 MHz-2620 MHz2570 MHz-2620 MHzTDD391880 MHz-1920 MHz1880 MHz-1920 MHzTDD402300 MHz-2400 MHz2300 MHz-2400 MHzTDD
Note that some bands overlap, e.g., band 2 and 36, meaning that carrier frequencies within 1930-1990 MHz can be used for both FDD and TDD (although not in the same geographical area). Countermeasures against UL interference will be required by the UE whenever TDD cannot be ruled out. Otherwise, the UE may mistake other UEs for eNBs, and the cell search will fail or take a very long time.
One method of UL interference mitigation during initial cell search on a TDD carrier (initial or inter-frequency) is to estimate the interference in the filter output, and scale any correlation peak down by the interference (i.e., the correlation peak value is divided by the interference estimate). The interference is estimated in an interval spanning about half a subframe on each side of the analyzed time instant. This approach may require high complexity in terms of memory and/or computational demands, depending on implementation. For a computationally efficient implementation, large buffers are necessary to keep track of the samples entering and leaving a window for interference level estimation.
Furthermore, this approach assumes a substantial number of samples will be present, in addition to the interval over which the cell search is conducted. In initial cell search this is not a problem since the UE is not mandated to find a cell within a particular time span, and hence the UE can collect as many samples as needed. However, when conducting inter-frequency cell search the first time on a TDD carrier, the timing (synchronization) will not be known, so measures to deal with UL interference are required. At the same time, there are only a limited number of samples available in the 6 ms transmission gap. Radio frequency switching will consume parts of this gap, and in the standard, it is only assumed that an efficient gap of 5 ms will be available. Therefore, it is not possible to use a large number of samples in addition to the ones to be examined, since most of the samples are needed in the search for cells.